1. Field of the Invention:
This invention relates to method and apparatus for manufacturing portland cement. In current embodiments, it relates to a method and apparatus for manufacturing portland cement by way of a rotary kiln. In one aspect, it relates to obtaining enhanced control over the quality and specifications of cement clinker used for the cement.
2. Description of the Prior Art:
The prior art has seen many different approaches to the manufacture of cement, ranging from the earliest recorded history to modern day methods of manufacturing portland cement. The conventional processes in this country for manufacturing cement have generally comprised admixing predetermined proportions of calcareous and argillaceous, or siliceous, materials. The materials may be admixed with water to form a slurry or may be dried to form a dry, raw meal. In any event, the admixture is continuously fed into and burned within a cement forming kiln to form clinker. The resulting clinker is mixed and pulverized with gypsum to form dry powdered modern day portland cement.
The calcareous material may consist of limestone, marl, chalk, oyster shells and the like. The argillaceous material may consist of clay, shale, slate, slag, fly ash, sand and the like. The proportions of these ingredients in the mixtures determines the resulting chemical composition of the clinker and the finished cement. Additives may be blended with one or more of the mixtures to provide the cement characteristics.
The different types of portland cement are well recognized and defined by the American Society of Testing Materials (ASTM). The most commonly manufactured cement is known as Type I, or general purpose cement. The remaining four types, Types II-V, are referred to as special purpose cements. These special purpose cements differ in the proportions of respective ingredients and are usually manufactured in lesser quantities.
To try to provide one or more of the features delineated hereinafter as desirable, a wide variety of artifices have been resorted to in the prior art. For example, issued U.S. patents as early as 1883, illustrated by U.S. Pat. No. 274,288, have disclosed addition of fuel with the argillo-calcareous raw materials for making cement. Many patents have issued since then with a similar idea. Such widely separated patents as U.S. Pat. No. 1,728,828 and U.S. Pat. No. 3,589,920 describe and claim the introduction of calcareous material into the burning zone of the kiln to form a satisfactory cement composition. Patents such as U.S. Pat. No. 2,477,262, describe and claim the classic insufflation process. Patents such as U.S. Pat. No. 3,437,329, describe introducing special additives into the combustion zone in which the fuel, such as coal, is burning to produce special purpose cements.
The latter patent has a good description of the need for flexibility in being able to change composition of the cement rapidly to eliminate the requirement for separate kilns for producing special purpose cement, as well as the other advantages inherent in the process described therein.
In our previously filed and co-pending application Ser. No. 546,271, entitled "Cement Production Having High Degree of Flexibility", filed Feb. 3, 1975, now abandoned, we described a method of making cement having a plurality of features not theretofore provided; and the descriptive matter of that application is embodied herein by reference for the details contained therein. Briefly, that invention included the use of coarsely ground calcareous material being fed into the feed end of the kiln with the complementary and finely ground portion of the material, including the siliceous material, being intimately admixed with the particulate fuel and fed into the combustion zone therewith for enhanced chemical activity and high temperature combustion. That invention enabled achieving significant advantages over the prior art theretofore, but still was not a completely perfect process.
Despite the wide variety of approaches tried, the prior art has not achieved a plurality of the desirable features delineated hereinafter.
The desirable features are:
1. The apparatus and process should allow fine control over the quality and specifications of the cement clinker even while achieving a plurality of the other features delineated hereinafter.
2. The apparatus and process should be flexible and be able to manufacture cement of different compositions readily without necessitating long periods of time to change over the operation of the kiln and related accessories and without requiring special purpose kilns.
3. Very importantly in the face of the current cost and scarcity of energy resources, the method and apparatus should be more efficient and enable producing cement with a lower expenditure of energy than heretofore.
4. The method and apparatus desirably would have the combined advantages of the wet process of cement manufacture with the low heat loss and efficiencies closer to that of a dry process, without significant capital expenditure.
5. The process and apparatus desirably obtains enhanced chemical activity. Producing cement is notoriously slow. The reactions are now analogous to chemical reactions in which kinetics are controlled by solid state diffusion. Consequently, finely ground siliceous, or argillaceous and calcareous feed materials have been required. This was particularly burdensome but the experts have maintained that there is a necessity for particle sizes less than 50 mesh if satisfactory cement clinker is to be produced in economically feasible residence times in the kiln.
6. The method and apparatus should reduce the expense of having to finely comminute, or grind, all of the feed material fed into the feed end of the kiln.
7. The method and apparatus should enable converting a conventional plant to this invention with relatively minor capital investment.
8. The method and apparatus should enable increasing the capacity of existing plants following conversion to this invention.